The Gaia parallax discrepancy for the cluster Pismis 19, and separating $\delta$ Scutis from Cepheids

TL;DR

This study addresses discrepancies in Gaia parallaxes for Pismis 19, establishes a red clump distance, confirms a variable star's membership, and introduces a criterion to distinguish $\delta$ Scuti stars from Cepheids based on period and Wesenheit Law break.

Contribution

It provides a new distance measurement for Pismis 19, confirms membership of a variable star, and proposes a method to differentiate $\delta$ Scuti stars from Cepheids using period and Wesenheit Law features.

Findings

Gaia parallaxes for Pismis 19 are inconsistent with red clump distances.

The variable star OGLE GD-CEP-1864 is likely a $\delta$ Scuti star.

A criterion based on period and Wesenheit Law break separates $\delta$ Scutis from Cepheids.

Abstract

Pre-Gaia distances for the open cluster Pismis 19 disagree with Gaia parallaxes. A 2MASS $JK_s$ red clump distance was therefore established for Pismis 19 ($2.90\pm0.15$ kpc), which reaffirms that zero-point corrections for Gaia are required (e.g., Lindegren et al.~2021). OGLE GD-CEP-1864 is confirmed as a member of Pismis 19 on the basis of DR3 proper motions, and its 2MASS+VVV color-magnitude position near the tip of the turnoff. That $0^{\rm d}.3$ variable star is likely a $\delta$ Scuti rather than a classical Cepheid. The case revealed a pertinent criterion to segregate those two populations in tandem with the break in the Wesenheit Leavitt Law ($\simeq 0^{\rm d}.5$). Just shortward of that period discontinuity are $\delta$ Scutis, whereas beyond the break lie first overtone classical Cepheids mostly observed beyond the first crossing of the instability strip.